Jupiter’s Satellites and Saturn’s Rings


Johannes Hevelius, Selenographia (Danzig, 1647), Fig. G.

This illustration from Edward Worth’s copy of Johannes Hevelius’s Selenographia (Danzig, 1647), depicts the planets Saturn, Mars and Jupiter. Jupiter’s four ‘Medicean’ moons are clearly visible, as are three different views of Saturn’s rings. Both the satellites of Jupiter and strange formations around Saturn had first been identified by Galileo in 1609 and in particular Jupiter’s satellites have become synonymous with his name ever since he first drew attention to them on the title page of his Siderius Nuncius (Venice, 1610).

Jupiter’s Satellites: the Four Medicean Moons

Galileo’s decision to name his new discovery after the powerful Medici family of Florence was an inspired bid for patronage. From his first sighting of them on the night of 7 January 1610 until at least 1612 he was obsessed by making tables of their positions. As Van Heldin (2003) relates, one reason for his intense interest was because he recognised their application for the problem of longitude. His first sighting had shown three small stars – two to the east and one to the west of the planet, but as his observations progressed over the next week he began to realise that they were changing position – and that that they were, in fact, four satellites orbiting the planet. The following illustration is from Worth’s copy of the Florentine edition of Galileo’s works in 1718 and depicts the various positions of the ‘Medicean Moons’:


Galileo Galilei, Opera (Florence, 1718), vol 2, pp 32-33.

We can compare this with similar information, this time from Worth’s copy of Hevelius’s Selenographia:


Johannes Hevelius, Selenographia (Danzig, 1647), Fig. KKK.

The satellites of Jupiter proved beyond doubt that planets could have satellites. The similarity between the satellites of Jupiter and the Moon’s orbit of the Earth was not lost on Galileo and succeeding astronomers, and could be used as one argument among many to support (if not officially prove) the Copernican system. For many it was a corroboration of something they had already come to be convinced of by the work of either Copernicus himself or the earlier observations of Tycho Brahe. Jean Picard’s trip to the site of Brahe’s Uraniborg in 1671 was made with the view of verifying the co-ordinates of the site so that Brahe’s observations could be compared with observations from Louis XIV’s observatory at Paris but while he was there he was also careful to take observations of the satellites of Jupiter as Brahe had died before Galileo’s great discovery. These he compared with observations made at Paris by Gian Domenico Cassini. Comparisons of sightings of the satellites was essential for studies of longitude and in 1707 we find the English astronomer William Derham requesting Samuel Molyneux of Dublin ‘to observe the eclipses of Jupiter’s satellites, or at least so many of them as that you and I may settle the exact difference of longitude between Dublin, Upminster, London and other places. If you could observe most of them and send me your observations, you will do a very acceptable service in order to correct the theories of the satellite motions, that we may have good tables of their eclipses and make use of them in finding the longitude.’* In return Derham sent Molyneux a copy of Flamsteed’s ‘table of eclipses of Jupiter’s satellites when they become visible after Jupiter is in opposition to the sun 1707’ and later sent him similar information for the year 1709.

Saturn’s Rings

Galileo’s observations of the planet Saturn noticed that there were odd shapes around the planet whose shape changed over time but it was not until fifty years later when the ground-breaking work of Christiaan Huygens solved the riddle of Saturn: Saturn had rings. Undoubtedly the advent of newer and higher powered telescopes in the middle of the seventeenth century enabled astronomers like Huygens and Cassini to make major discoveries and Huygens himself acknowledged this in his Systema Saturnium, first published in 1659, which forms part of a Lyon 1724 edition of his works purchased by Worth. Whereas Galileo’s strongest telescope had a magnification power of 30, that of Huygens had a power of 50 in 1655 (when he first viewed Saturn with club-shape on either side), and 100 in the following year when he first published his anagram suggesting his ring hypothesis. On p. 634 of Worth’s edition Huygens depicts earlier views of Saturn: including that of Galileo at the top (Fig 1 in the diagram):


Christiaan Huygens, Opera varia (Lyon, 1724), vol 2, p. 634

Galileo was aware that there was something strange about the planet – by 1612 he was alluding to the fact that the ‘two small stars’ which he had seen in 1610 had disappeared but was strongly of the opinion that they would be seen again (clearly thinking that they were orbiting the planet). Though a number of other sightings were made, as the above illustration makes clear, the more sightings there were, the more perplexing the problem became. It was certainly not helped by the fact that twice during Saturn’s thirty year orbit around the Sun the rings become invisible. Despite the fact that Hevelius made a specific observation of Saturn, publishing at the same time as Huygens was beginning to realise the ring theory and just three years before the publication of the Systema Saturnium, he was still of the opinion that he was dealing with three bodies. Huygen’s own depiction of Saturn is visible on p. 704 of the same volume – we can see here how he also draws attention to the relative sizes of the planets in the same plate:


Christiaan Huygens, Opera varia (Lyon, 1724), vol 2, p. 704.


It was an iconic image and remained the dominant one as Worth’s copy of David Gregory’s remarkably similar rendering in 1702 demonstrates:


David Gregory, Astronomiae, physicae & geometricae elementa (Oxford, 1702), p. 390.

Here we see that, despite Cassini’s discovery in the intervening period that Saturn’s Ring was not continuous, Gregory continues on the iconographic tradition of Huygens.

Dublin and Huygens

The Papers of the Dublin Philosophical Society, published by the Irish Manuscript Commission, give us vital information concerning astronomical activity both in Dublin in the period 1684-1709 (and elsewhere, due to the friendship networks of the members of the Society). An excellent example is Thomas Molyneux’s account of his visit to Huygen’s observatory in 1684. Molyneux, a Dublin physician like Worth, was a brother of the even more illustrious William Molyneux, whose Sciothericum Telescopicum was printed at Dublin in the 1686 and subsequently bought by Worth.

Writing to his brother William from Leiden on 15 August 1684, Thomas recounts his adventure as follows:

‘The 7th of this month, having the opportunity of a gentleman my acquaintance’s company, I made the other journey to The Hague, in hopes to see Mr Huygens, which I did, and I was received extraordinarily civilly by him. After some discourse, understanding that I was an Englishman, he, beyond my expectation, talked to me in my own language, and pretty well. He carried me up into his study, where he showed me a most curious mechanical movement of his own contrivance…. it stands up against the wall like a clock. The outward dial-plate, where the great circle of the ecliptic is described, containing within it all those of the planets, placed according to the hypothesis of Copernicus, is about two foot square. This shows you at once the minute, hour, day of the month and year, with the exact postures and aspects that all the planets bear to the sun and one another at that very moment, and also the site of the satellites in respect of their middle planets, viz. the three of Mars and the four of Jupiter – these and all the planets obsolve [sic] their course, just in the same time as they do in the heavens. Here you have all retrogradations, eccentricities, and other irregular motions described. He told me that in 1682, when Saturn and Jupiter were in conjunction several times by their retrograde motion, that they were so just as often in his machine as in the heavens. He has not here (for he could with convenience) made the planets, and the diameter of these circles they describe, answer according to that proportion they truly bear to one another; but this he had done by a smaller scheme in one corner of his plate, according to the latest observations he and the Parisian astronomers could make. He complained much that the motion of Saturn was not yet certainly regulated by any of the astronomers…’*

Huygens was indeed interested in the relative size of the planets as a number of his illustrations demonstrate. In his above illustration of the Ring of Saturn we can see the relative size of Earth and the Moon and in the following illustration he compares the relative sizes of all the planets:


Christiaan Huygens, Opera varia (Lyon, 1724), p. 712.

Molyneux’s account likewise brings to mind another useful comparison given by Huygens: his diagram of the orbits of moons of Earth, Saturn and Jupiter:


 Christiaan Huygens, Opera varia (Lyon, 1724), p. 700.

Selected Reading:

*Quotations marked with an asterisk may be found in Hoppen, K. T. (ed.) (2008) Papers of the Dublin Philosophical Society 1683- 1709 (Dublin: Irish Manuscript Commission), 2 vols.
Débarbat, S. and Wilson, C. (2003). ‘The Galilean satellites of Jupiter from Galileo to Cassini, Römer and Bradley’ in in R. Taton and C. Wilson (eds.) Planetary astronomy from the Renaissance to the rise of astrophysics. Part A: Tycho Brahe to Newton. Cambridge: Cambridge University Press. Series: The General History of Astronomy, pp. 144-57.
Shapley, D. (1949). ‘Pre-Huygenian Observations of Saturn’s Rings’, Isis 40 no. 1., 12-17.
Van Helden, A. (2003). ‘Galileo, Telescopic Astronomy, and the Copernican System’, in R. Taton and C. Wilson (eds.) Planetary astronomy from the Renaissance to the rise of astrophysics. Part A: Tycho Brahe to Newton. Cambridge: Cambridge University Press. Series: The General History of Astronomy, pp. 81-105.
Van Helden, A. (2003). ‘The Telescope and Cosmic Dimensions’ in R. Taton and C. Wilson (eds.) Planetary astronomy from the Renaissance to the rise of astrophysics. Part A: Tycho Brahe to Newton. Cambridge: Cambridge University Press. Series: The General History of Astronomy, pp. 106-118.
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